Examples of polar lipids include phospholipids (e.g., phosphatidyl choline, phosphatidyl ethanolamine, phosphatidyl inositol, phosphatidyl serine, phosphatidylglycerol, and diphosphatidylglycerols), cephalins, sphingolipids (sphingomyelins and glycosphingolipids), lysophospholipids and glycoglycerolipids. Phospholipids are composed of the following major structural units: fatty acids, glycerol, phosphoric acid, amino alcohols, and carbohydrates. They are generally considered to be structural lipids, playing important roles in the structure of the membranes of plants, microbes and animals. Because of their chemical structure, polar lipids exhibit a bipolar nature, exhibiting solubility or partial solubility in both polar and non-polar solvents. The term polar lipid, within the present description, is not limited to natural polar lipids but also includes chemically modified polar lipids.
One of the important characteristics of polar lipids, and especially phospholipids, is that they commonly contain polyunsaturated fatty acids (PUFAs), fatty acids with two or more unsaturated bonds. In many plant, microbial and animal systems, they are especially enriched in the highly unsaturated fatty acids (HUFAs), fatty acids with 4 or more unsaturated bonds, of the omega-3 and omega-6 series. Although these highly unsaturated fatty acids are considered unstable in triacylglycerol form, they exhibit enhanced stability when incorporated into phospholipids. A primary source of HUFA and/or PUFA-rich polar lipids is egg yolk. Several processes are used for the recovery of egg phospholipids on an industrial scale.
Previous methods to separate polar lipids, which include phospholipid-containing materials, from native biomaterials have been disclosed in WO 01/76715, “Method for the Fractionation of Oil and Polar Lipid-Containing Materials.” Other disclosures include International Patent Publication No. WO 01/76385 and Canadian Patent No. 1,335,054. These disclosures teach a number of different processes regarding the separation of non-polar/neutral lipidic compounds or oils (triacylglycerides, cholesterol, pigments, hydrocarbons, etc), from polar lipidic compounds (phospholipids, cephalins, sphingomyelins, etc) by the use of water and organic soluble solvents (ethanol, isopropanol, etc.), aided by the force of centrifugation and differences in densities, combined with the degree of solubility and partition between the aqueous, organic fraction and the oil and the residual, solid fraction (insoluble proteins, ash and carbohydrates). These processes, although practical, require the use of high concentrations (greater than 50%) alcohol in some or all steps which can add costs and decrease efficiency of partitioning between neutral and polar lipids, as neutral fats are more soluble in high concentrations of alcohol. Accordingly, processes to separate phospholipids from other components still have possibilities for improvements, such as the minimization of unit operations or processing steps, as well as reduction in the cost of final production with improved yield and purity of the polar lipids.
Other processes used in the past include the use of a combination of organic solvents such as hexane, acetone, isopropanol and ethanol to extract the neutral fats using non-polar solvents from dry egg yolk, while the more polar solvents have been used to further extract the polar fractions from the yolk-extracted residue. The previous processes have several drawbacks, including the use of organic solvents (i.e. hexane and acetone) that have a higher toxicity than ethanol or isopropanol. Also, some processes require a dry matrix (egg yolk powder) to start the process, which subjects the egg to a heat treatment, lowering the quality of the starting material. This drying step also adds to the cost of the final product. In addition, the large amount of solvent needed to completely extract the oil from the matrix and subsequent polar materials is costly.
In other cases, to avoid the use of hexane or acetone, numerous processes have been proposed involving the use of supercritical fluids, especially supercritical CO2. For example, U.S. Pat. No. 4,367,178 discloses the use of supercritical CO2 to partially purify crude soy lecithin preparation by removing the oil from the preparation. However, supercritical fluid extraction systems are very expensive and cannot be operated continuously. Further, extraction times are long and the egg yolks or other biomaterials typically must be dried before extraction, and this increases the difficulties of stabilizing the starting dry product with antioxidants. All of these factors make the supercritical process one of the most expensive options for extracting and recovering polar-lipid material or mixtures of these materials, and although it is an elegant solution, supercritical CO2 still only removes the neutral lipids from the matrix, leaving behind the polar lipids entrained with the proteins, etc., which will need subsequent extraction with a polar solvent.
Thus, there remains a need for improved processes for the extraction and separation of polar lipids from polar lipid-containing material.